398 HARRIS HAWTHORNE WILDER ON 



are very variable in their mutual relations. Occasionally all three will be distinct and of 

 about the same size, but usually the venti-al spine is much smaller than the other two. 

 There may also be varying degrees of bifurcation of the genuine transverse process, or it 

 may present one or more small processes in addition to the usual three. 



Vertebra hearing the first haemal arch. — This is normally the 23d in the series, but 

 it may Ije the 22d or even frequently the 24th. After the sacrum, the neural plates rap- 

 idly narrow, and. by the time the vertebra in question is reached, this part has become 

 about as narrow as the base of the neural spine, so that the latter appears in direct con- 

 tinuation with it. the posterior zygapophyses appearing as lateral processes borne upon 

 the sides of the spine. This increase of the apparent length of the neural spine renders 

 it strikingly similar to the haemal spine, and thus is seen the first suggestion of that 

 dorso-ventral bilaterality which becomes so marked in the more posterior caudal vertebrae. 

 The transverse process is thin and poorly developed, and the occasional bifurcation of its 

 free end gives an indication of its division into tubercular and capitular portions as seen 

 in the vertebrae immediately posterior to this. The haemal arch, which is a new element 

 and thus gives distinction to this and the following vertebrae, begins as a pair of low, thin 

 ridges springing from the ventral side of the centrum and uniting across the mid-ventral 

 line near the posterior limit of the vertebra in such a way as to frame in a haemal 

 foramen just ventral to the centrum, which, with the foramina of the succeetUng verte- 

 brae, forms the haemal canal. The ^^entral laminae are perforated as usual by the two 

 ventral foramina, and, as they are nearly covered by the two ridges that form the roots 

 of the haemal arch, a pair of foramina occurs also in these latter opposite the usual 

 ones. In most instances the first haemal arch appears, as it were, suddenly and perfectly 

 developed, and the vertebra immediately preceding the one that bears it usually shows no 

 suggestion of such a structure. Occasionally, however, a rudiment of the foot or root of 

 the arch appears upon one or both sides of the previous vertebra in the form of a thin 

 plate lying parallel to the ventral lamina, and extended across from centrum to outer end 

 of the transverse process, and, in one instance observed, a slender haemal process appeared 

 upon the left side, which projected back over the following vertebra and plainly repre- 

 sented one side of a rudimentary haemal arch. There was no trace of this upon the 

 other side. 



The caudal vertebrae posterior to the above, with especial reference to the terminal 

 ones. — The most conspicuous characteristic of the caudal vertebrae posterior to the above 

 is the presence of nearly equal neural and haemal arches running out into long spines 

 with their free ends pointed obliquely backwards. The reduction of the transverse 

 process, which begins just posterior to the sacrum, progresses rapidly after passing the 

 first haemal arch. The process first becomes divided into its two portions, the dorsal one 



